1. Field of the Invention
The present invention relates to a positioning apparatus for aligning a workpiece with a tool electrode in an electric discharge machine and, more particularly, to a contact sensing apparatus in an electric discharge machine, comprising a tool electrode and a workpiece which are moved relative to each other to bring a tool electrode surface into contact with a workpiece reference surface, thereby discriminating a position of the workpiece relative to the position of the tool electrode in accordance with a contact point.
2. Description of the Related
FIG. 1 is a circuit diagram of a conventional contact sensing apparatus in an electric discharge machine. Reference symbol P denotes a tool electrode; W, a workpiece as an object subjected to machining; and E, a machining power source for generating a discharge current between the tool electrode P and the workpiece W. The machining power source is not associated with the scope of the present invention, and a detailed arrangement of the circuit will be omitted. Reference symbol D1 denotes a diode; R1 and R2, resistors, respectively; and C1, a compactor. In the conventional contact sensing apparatus in FIG. 1, a voltage of -100 V, -200 V or the like is applied to the tool electrode P from the machining power source to perform machining. During machining, the diode D1 blocks a voltage applied to the tool electrode P such that the voltage is not applied to the comparator C1. When the tool electrode P is positioned relative to the workpiece W, a low voltage -V (e.g., -5 V or -15 V) is applied through the resistors R2 and R1 and the diode D, while the tool electrode P and the workpiece W are moved relative to each other. When the tool electrode P and the workpiece W are brought into contact with each other, an input to the comparator C1 changes through the diode D and the resistor R1. This change is compared with a reference voltage VL1 so as to detect a contact between the tool electrode P and the workpiece W. As a result, the relative positions of the tool electrode P and the workpiece W are detected.
However, since the tool electrode P and the workpiece W are brought into contact with each other while the low voltage is applied therebetween, rusting or the like occurs on the contact surface of the workpiece W. In order to detect the relative positions of the tool electrode P and the workpiece W, the tool electrode P and the workpiece W are several times repeatedly brought into contact with each other. However, when rusting or the like occurs on the contact surface of the workpiece W, electrical contact between the tool electrode P and the workpiece W becomes poor, so that repeated measurement precision is degraded. When the workpiece W comprises a carbide, cobalt contained in the carbide serves as a binder. The carbide tends to become damaged since it has an electrolytic property. In particular, when the carbide is dipped in water, the electrolytic reaction speed is increased. When a working liquid comprises water, the tool electrode P and the workpiece W are brought into contact with each other in water during positioning, thereby damaging the workpiece W since it is subjected to the electrolytic reaction.
When the tool electrode P and the workpiece W are for some reason kept in contact with each other for a long period of time during positioning, the workpiece W is electrolytically reacted and becomes scored, i.e., the surface of the workpiece W will have lines, marks, gauges, grooves, or other roughening. In order to prevent this, the tool electrode P and the workpiece W must be brought into contact with each other in a dry state. However, when a surface portion of the workpiece W is machined and subsequently another surface portion or another workpiece is subjected to alignment for machining, water must be removed from the workpiece W and the tool electrode P, thereby degrading the working efficiency. In addition to this disadvantage, the electric discharge machine itself must be modified in accordance with the presence/absence of water. It is desirable to align the workpiece and the tool electrode in the presence of water when the workpiece is actually machined.
In addition to the carbide workpiece, a molybdenum or chromium workpiece or the like is subject to electrolysis due to contact between the tool electrode P and the workpiece W at the time of position alignment. When an aluminum workpiece W is used, an insulating film is formed on its surface due to electrolysis. As a result, positioning cannot be properly performed, resulting in inconvenience. Furthermore, when machining is interrupted, as shown in FIG. 1, the low voltage -V is constantly applied between the tool electrode P and the workpiece W, and the workpiece W made of an above-mentioned material is subject to electrolysis. As described above, when the aluminum workpiece is used, an insulating film is formed thereon, thus disabling reworking.
In a wire cut electric discharge machine, a wire electrode P is kept taut, and the workpiece W and the wire electrode P are brought into contact with each other for positioning. However, in this case, a Coulomb force is generated to move the wire electrode P by a distance of about 2 to 3 microns. As a result, high precision positioning cannot be performed.